(1) Field of the Invention
This invention relates to a grinding sheet for grinding glass lenses such as optical lenses, especially suitable for precise grinding to be conducted after rough grinding, and a process for the preparation thereof.
(2) Description of the Prior Art
A typical process for the production of optical lenses comprises the step of roughing a pressed glass material by using a machine called "curve generator" provided with a cut-type diamond grinding wheel, the sand-grinding step of wet-lapping the roughly ground material with an abrasive (abrasive sand) having a grain size fo about 10 .mu.m and the polishing step (i.e., mirror polishing step) using a polishing powder of cerium oxide and a polishing pad of a foamed polyurethane sheet or a felt. In principle, these three steps are similarly adopted for the production of spherical spectacle lenses and astigmatic spectacle lenses and other lenses such as camera lenses.
At the sand-grinding step, a diamond pellet has recently been used instead of the abrasive sand for increasing the operation efficiency, improving the working environment and simplifying the disposal of the sludge of the abrasive. Since this step does not use an abrasive sand, the step is called "precision grinding step" or "smoothing step". The diamond pellet is a common name for a diamond tool in the form of a tablet which is formed by mixing a metal powder with a diamond powder, compression-molding the mixture and then sintering the molded mixture and is ordinarily used in the form having a diameter of 10 to 20 mm and a thickness of about 3 mm. When this diamond pellet is employed, since the hardness of diamond is higher than that of an fused alumina abrasive customarily used at the sand-grinding step, even if the tool is rotated at a high speed, scattering of the abrasive as observed at the conventional sand-grinding step is not caused, and the grinding efficiency is increased. Furthermore, at the conventional sand-grinding step, since the curvature of a cast iron dish wheel changes in a short time due to wearing, it is necessary to often correct the surface. In contrast, in a dish wheel to which diamond pellets have been bonded, since the degree of wearing is low, the frequency of correction of the surface is reduced. Furthermore, in case of the production of spherical lenses, if optimum conditions for uniform wearing can empirically be found out by adjusting the size of the dish wheel, the manner of bonding the diamond pellets and the position of a so-called spindle for forcing a lens against the dish wheel, the grinding operation can be continued for a long period of time without correcting the surface even if wearing of the diamond pellets is caused. Moreover, in case of the diamond pellets, there is no need of disposal of a sludge.
As is seen from the foregoing description, the grinding operation using a diamond pellet is advantageous over the conventional sand-grinding operation, but the grinding operation using a diamond pellet is not suitable for the production of special lenses such as astigmatic spectacle lenses for the reasons described below.
(1) A diamond pellet is compression-molded at the ordinary molding step in course of preparation while predetermined curvatures are given to the upper and lower surfaces, and it is then sintered. However, the precision for these curvatures is not so high as required for astigmatic spectacle lenses. Accordingly, after the diamond pellets have been bonded to a dish wheel, it must be mutually lapped with a standard dish wheel by using an abrasive sand, before the abrasive dish wheel is used for smoothing. Furthermore, since the surface of an astigmatic spectacle lens is complicated, a long period of time is necessary for this mutual lapping operation.
(2) The processing for formation of the surface of an astigmatic spectacle lens is different from the rotary operation for formation of a spherical spectacle lens and it is difficult or even impossible to find optimum conditions for uniform wearing by adjusting the size of the dish wheel or the position of a spindle and the surface precision changes as the processing operation is continued. In case of a diamond pellet which has a certain thickness, correction of the surface should inevitably be performed before the diamond pellet has been worn away, and therefore, continuation of the operation is disturbed and there arises a problem concerning the stability of the product. If the diamond pellet is discarded without performing the correction of the surface, since diamond is expensive, the process becomes economically disadvantageous. However, a very thin pellet is prepared by sintering, and therefore, the productivity is reduced.